Hurricane Sandy has moved on after wreaking havoc on the East Coast and leaving several mid-Atlantic cities underwater. New York and New Jersey bore the brunt of Sandy’s fury and millions are now dealing with the dangerous consequences. In addition to being without power, millions in the Northeast now face another threat: the backed up municipal sewer system is spewing a toxic stew of raw sewage, industrial chemicals and floating debris into streets and neighborhoods.

Although sewage overflows are a chronic problem in the Hudson River watershed, experts say that the 13-foot storm surges caused by Hurricane Sandy have created an unprecedented health threat. What’s different about the sewage contamination from Sandy is that, due to the storm surge, sewage spilled back into roads and homes in many communities, rather than being discharged into the river or harbor, according to a release from Riverkeeper, a New York-based clean water advocacy group.

“This is like an Exxon Valdez spill from nonpoint sources,” said Capt. John Lipscomb, who weathered the storm aboard Riverkeeper’s patrol boat, the R. Ian Fletcher. “The amount of pollution released by this storm is staggering. Instead of it being one product like crude oil, it’s a thousand different products and floatables, and instead of being from one source like a tanker, it’s from a thousand different locations.” Although rain and ocean water have diluted the sewage greatly, it’s still extremely dangerous and should be treated as an immediate health threat.

If you live in an area where there’s still standing water in streets, basements, or yards, do everything you can to avoid it. Resist the urge to wade into the water, even if it’s to check on damage or clean-up progress. One particular area of concern is the Gowanus neighborhood in Brooklyn, which abuts a 1.8-mile canal that was recently designated a Superfund cleanup site by the U.S. Environmental Protection Agency due to a legacy of industrial pollution and sewage discharges.

“If you live near the canal, do not touch standing water in the area, or any sediment or debris left by Gowanus flood-waters,” wrote City Councilmember Brad Lunder in an email late Tuesday. “After the storm, the EPA and DEP are committed to work together to conduct any sampling needed to address potential issues of toxicity created by the flooding.” Unfortunately, many of his constituents were still without power or internet on Tuesday, and many saw the email after they’d already come into contact with the water.